(1) Field of the Invention
This invention relates to a case shift mechanism assembled in a keyboard portion of a printer or the like machine. More particularly, the present invention relates to a case shift and lock input mechanism wherein case shift operation is governed by selectively closing a single shift control switch.
(2) Description of the Prior Art
Keyboards of the kind used in conjunction with printers or typewriters commonly employ shift function keys comprising a pair of shift keys and a shift lock key. The three keys are individually selectable by the typist for typing, e.g., one or a few upper case characters by depressing one of the shift keys or a series of upper case characters by depressing the shift lock key which is then held in its depressed position by a locking device.
The prior art discloses many varied and different input mechanisms for accomplishing case shift and lock functioning. These mechanisms must economically combine ease of part fabrication and assembly for low manufacturing expense. This, coupled with high reliability and ease of maintenance at low cost are becoming increasingly difficult to satisfy due to ever increasing labor and material costs. Consequently, use of unduly complex mechanisms requiring close dimensional tolerances and careful adjustments during assembly are impractical.
With the advent of electro-mechanical typewriters, manufacturers have successfully reduced many of the previously complicated linkages found in totally mechanical prior case shift mechanisms by replacing them with simple electronic components, such as switches. An example of an electro-mechanical input combination utilized for shift and shift lock functioning is found in U.S. Pat. No. 4,071,719 granted on Jan. 31, 1978 to Robert C. Madland. This device shows two shift keyswitches, each having a slide member coupled together by a rod. A shift lock keyswitch has its own slide member. The slide member of the lock keyswitch interacts with the slide members of the two shift keyswitches so the slide linkage will not operate except when the shift keyswitch is in the lock mode. Each keyswitch has an associated switch for performing a case shift operation. The disadvantages in this device are directly related to its overall complexity, as for example, to the relatively large number of components needed, including the use of a plurality of expensive electrical switches for shift operation. The phsycial configuration of the keyswitches themselves are complex making them difficult to fabricate and assemble.
Another example of a shift input utilizing an electromechanical combination is disclosed in U.S. Patent No. 3,863,748 granted to Richard Trezise on February 4, 1975. The Trezise patent discloses a shift lock mechanism comprising a spring supported lock key and a latching bellcrank for holding the lock key when it is fully depressed. A reed switch is provided to coact only with a shift key when it is depressed to initiate shift operation. To effectuate shift operation from the lock key, the shift key must also be depressed. To accomplish this, Trezise provides a plate extending from the shift key beneath the lock key for engageably causing the shift key to move with the lock key. A major disadvantage found in this arrangement is that dissimilar key forces resisting key movement, are expierenced by the operator when depressing the shift lock key as compared to depressing the shift key. Depression of the shift key is resisted only by its support spring. Depression of the lock key is resisted by its support spring together with the support spring of the carried shift key. Thus, two different key forces must be overcome by the typist when selwecting between shift and shift lock inputs.
Accordingly, there is a need for a more economical, highly reliable case shift and lock input mechanism. Moreover, an improved shift key mechanism must incorporate a minimum number of parts arranged in a manner to provide substantially uniform key touch, thus, avoiding disadvantages found in the prior art.